1. Field of the Invention
The present invention relates to an image forming apparatus that transfers a toner image on an image carrier to an intermediate transfer member or a recording material, and also relates to an image forming apparatus that transfers a toner image on the intermediate transfer member to the recording material.
2. Description of the Related Art
To date, an image forming apparatus has been known that can form color images by performing a primary transfer and a secondary transfer as follows: in the primary transfer, toner images are formed on image carriers and are sequentially transferred to an intermediate transfer member; and in the secondary transfer, color images each comprising a plurality of toner images are formed on the intermediate transfer member, and thereafter these color images are collectively transferred to a recording material.
In such an intermediate transfer type image forming apparatus, the intermediate transfer member is not susceptible to scratches, and dust thereon such as paper powder can be easily cleaned off as compared with a photoconductive drum. Therefore, this type of image forming apparatus has the advantage of being adaptable to various recording materials, such as a cardboard, for example, having a basis weight on the order of 300 g/m2.
Trends toward higher quality images in an image forming apparatus require importance to be placed on its adaptation not only to ordinary paper but also to recording materials having improved surface smoothness (i.e., having small surface roughness), such as art paper and coated paper.
However, such an image forming apparatus has a problem of being incapable of sufficient transfer to a cardboard material having a surface with bumps and dips provided thereon, such as embossed paper or debossed paper.
As a conventional art for preventing transfer defects due to the surface roughness of a recording material, for example, Japanese Patent Laid-Open No. 11-24443 discloses an arrangement in which, with regard to the electrostatic force that acts on a toner image when performing a transfer operation from an image carrier to a recording material, an electrostatic attractive force or an electrostatic repulsive force is selectively used in accordance with a surface roughness condition of the recording material.
However, the above-described art does not completely overcome transfer defects due to surface roughness.
The occurrence of a transfer failure due to surface roughness is attributable to a phenomenon that, when a transfer current is applied in a state where a toner image on an intermediate transfer member and a recording material are not sufficiently adhered to each other, an abnormal electric discharge occurs in a gap between the toner image and the recording material, thereby hindering the toner from being normally transferred. Therefore, simply changing a method for applying a transfer electric field provides no basic solution to the problem.